Absorbent articles such as diapers, incontinence pads, sanitary towels or the like are gradually becoming thinner and thinner. Thin articles are in most cases more flexible and more discreet to wear and are therefore seen as something positive by users. Moreover, they are more compact, which is of course an advantage for transport and storage.
In order for it to be possible to offer thin absorbent articles with retained functioning as far as absorption capacity and leakage security are concerned, the use of different types of highly absorbent materials, what are known as superabsorbents, has become common. Such materials are polymers which are available in the form of particles, fibers or granules and are advantageously incorporated into a fibrous structure, in most cases comprising cellulose fluff pulp.
When superabsorbents are incorporated into a cellulose fibrous structure, however, a number of problems arise.
In order to retain the superabsorbent in the cellulose fluff structure, it is preferable for the fluff structure to be compressed to a high density of the order of 0.1-0.5 g/cm3. Such highly compressed structures feel hard and uncomfortable when they are used in an absorbent article.
Furthermore, fibers attach themselves to rollers and other machine parts during manufacture of the fibrous structures, which results in operational problems.
Moreover, liquid reception and liquid-spreading in a highly compressed material are very slow, which is a particular problem in absorbent articles intended to receive and absorb large quantities of liquid in a short period of time. For example, articles intended for incontinent adults must be capable of taking up several hundred ml of liquid which is discharged at great pressure in the course of ten seconds or so. Instead of being absorbed by the absorbent article, it therefore sometimes happens that the liquid instead flows out over the surface and runs out of the article along the edges and causes leakage. In practice, such an absorbent article may therefore have an actual absorption capacity which is considerably lower than might have been expected from theoretical calculations based on the absorption capacity of the component materials. In order to remedy this problem, use is often made of special liquid-receiving and/or spreading layers. However, such layers increase the thickness of the articles and contribute to a higher production cost owing to greater material consumption and a more complicated manufacturing process.
A modified cellulose fluff pulp produced by means of an oxidation process using nitroxide is previously known from EP 1 245 722. The process increases the degree of “self-cross-linking” between adjacent cellulose fibers and within one and the same cellulose fiber. In this way, an improved network for superabsorbents which are mixed with the modified cellulose fluff pulp is obtained in relation to conventionally cross-linked cellulose fluff pulps.
WO 00/66833 describes another type of modified cellulose fluff pulp where the fibers are provided with a coating of bentonite which prevents bonds between the fibers and reduces the number of fiber knots in the material.
However, a major problem with the treated cellulose fluff pulps is that fiber webs produced from such cellulose fluff pulps have much too low a mat strength to be capable of being handled in continuous manufacturing processes. Moreover, their capacity for retaining superabsorbent particles is still inadequate.
There is therefore a need to produce an absorbent structure with good processability, a high absorption capacity, a good liquid-receiving capacity and great softness and flexibility. Another object of the invention is to offer an absorbent structure with a fiber network of cellulose fibers, which absorbent structure has a good capacity for retaining superabsorbents and for spreading liquid in the absorbent structure.